The present invention relates to an acoustic surface wave filter for use in high-frequency circuits or the like of a radio communication apparatus and, more particularly, relates to an surface acoustic wave filter of a transversal type utilizing a unidirectional interdigital transducer and a radio communication apparatus using the same.
Surface acoustic wave filter (hereinafter called SAW filter) is a small and thin filter using Surface Acoustic Wave (hereinafter called SAW) excited on a piezoelectric substrate. The SAW filter is in practical use as a small and thin key component for mobile communication apparatuses such as mobile telephones.
SAW filters are broadly classified into transversal type filters and resonator type filters. When it is required to have a relatively broad bandwidth and flat phase characteristics in the pass-band as with IF filters for mobile telephones of Code Division Multiple Access (CDMA) system, SAW filters of the transversal type are being used. However, since a transversal type SAW filter produces a large insertion loss, there is a measure for reducing such a loss by adapting Interdigital Transducer (hereinafter called IDT) to be unidirectional.
Furthermore, unidirectional IDTs are broadly classified into those of multi-phase type and single-phase type. Especially, those of single-phase type are used widely because of its simple structure.
One of the prior art examples of the same is shown in FIG. 23A and FIG. 23B.
FIG. 23A is its top view and FIG. 23B is an enlarged view of a portion of FIG. 23A surrounded by an oblong solid line.
IDT shown in FIG. 23A and FIG. 23B is a kind of Electrode Width Controlled Single Phase Unidirectional Transducer (EWC-SPUDT). It is structured such that its basic component, when the wavelength of the SAW is denoted by xcex, has a total of three fingered electrodes 100a, 100b disposed in a region of one wavelength (xcex) on a piezoelectric substrate, of which one fingered electrode 100a has an electrode width of xcex/4 and two other fingered electrodes 100b each have an electrode width of xcex/8.
This EWC-SPUDT contains two reflecting electrodes 100a having their centers of reflection in asymmetrical positional relationship with respect to the center of excitation of SAW. Therefore, in one direction in which the excited SAW propagates, it is superposed in phase on the reflected wave and, in the opposite direction, it is superposed out of phase on the reflected wave. Thus, SAW is provided with a propagation directionality.
In the structure shown in FIG. 23A, a relatively strong unidirectionality is obtained in accordance with setting of film thickness of fingered electrodes 100a, 100b and number of pairs of the same, i.e., a directionality toward the right in the drawing is obtained. Further, such a SAW filter has been mainly used by inserting between frequency mixer 83 and IF amplifier 85 in receive system 220 in a transmit-receive circuit of a radio communication apparatus having transmit system 200 and receive system 220 as shown in FIG. 22.
Although such a conventional SAW filter has had a strong unidirectionality, its excitation efficiency has not been sufficiently good. Hence, there has been a problem that both of its insertion loss and its pass-band characteristics are not good enough to be used as a communication filter.
A SAW filter according to the present invention comprises at least two IDTs having four fingered electrodes disposed in a region on a piezoelectric substrate corresponding to one wavelength of a SAW excited on the piezoelectric substrate. At least one of the IDTs is formed of at least one pair of fingered electrodes having different electrode widths. The pair of fingered electrodes allows the SAW to obtain a unidirectional propagating characteristic on the same principle as in the aforementioned EWC-SPUDT.
A SAW filter having a sufficient unidirectionality, an excellent exciting efficiency and a sufficiently low insertion loss can be obtained by going through for example the following steps.
(a) Arranging two IDTs formed of pair of fingered electrodes having different electrode widths such that unidirectionalities are strengthened in the directions toward each other.
(b) Arranging two IDTs having the same shape in symmetrical positional relationship.
Further, by arranging an IDT including at least one pair of fingered electrodes having different electrode widths such that, in a region including the pair of fingered electrodes, which is one of the regions obtained by equally dividing the IDT having the pair of fingered electrodes such that a division corresponding to one-half wavelength of SAW includes the pair of fingered electrodes, the distance between the fingered electrodes (xcex3) is greater than the sum of the distance between the narrower of the fingered electrodes and an edge of the region near the same (xcex1) and the distance between the wider of the fingered electrodes and an edge of the region near the same (xcex2), the ripple inside the pass-band of the SAW filter can be reduced and attenuation outside the pass-band can be increased.
Further, when an IDT has both a pair of fingered electrodes being different in electrode width and a pair of fingered electrodes being the same in electrode width, it is arranged, in the former pair of fingered electrodes, such that the distance between the fingered electrodes (xcex3) is greater than the sum of the distance between the narrower of the fingered electrodes and an edge of the region near the same (xcex1) and the distance between the wider of the fingered electrodes and an edge of the region near the same (xcex2) and xcex1 less than xcex2. Then, the ripple inside the pass-band of the SAW filter can be reduced and attenuation outside the pass-band can be increased.
Further, pairs of fingered electrode may be arranged such that at least one IDT has a unidirectionality in one direction and at least one region of the IDT has a unidirectionality in the opposite direction. By arranging in such a manner, the size of the IDT can be made smaller than before. Thus, a SAW filter being of a small size and capable of producing a small insertion loss can be obtained.
Further, by taking any of the following steps (a) to (f), a SAW filter controlled for unidirectionality, controlled for the ripple due to insufficient unidirectionality or excessive unidirectionality, and provided with much improved filter characteristics can be obtained.
(a) Electrode width ratio of pair of fingered electrodes of at least one IDT is made different from electrode width ratio of at least one other pair of fingered electrodes of the IDT.
(b) Electrode width ratio of pair of fingered electrodes of at least one IDT is made different from electrode width ratio of at least one other pair of fingered electrodes of the IDT. Further, distance between electrodes of the former pair of fingered electrodes is made different from distance between electrodes of the latter pair of fingered electrodes.
(c) At least one IDT is adapted to have two kinds of pair of fingered electrodes being different in electrode width ratio.
(d) At least one IDT is adapted to have two kinds of pair of fingered electrodes being different in electrode width ratio and distance between electrodes.
(e) Metallization ratio of fingered electrodes is set between 0.4 and 0.7.
(f) Fingered electrodes are formed of aluminum or alloyed aluminum as main component and film thickness ratio is set between 0.005 and 0.035.
Further, a xe2x80x9828xc2x0-42xc2x0 rotated Y-cutxe2x80x99 quartz substrate may be used as the piezoelectric substrate to obtain a SAW filter having optimum temperature characteristics within the range of desired operating temperatures.
Further, a SAW filter of the invention may be disposed between a frequency mixer and an IF amplifier of a communication apparatus. Then, the number of amplifying elements of the amplifier can be decreased and power consumption can be reduced to obtain a communication apparatus with high performance at low cost.
A SAW filter as described above can provide sufficient unidirectionality, excellent SAW excitation efficiency and sufficiently low insertion loss, excellent amplitude and phase characteristics inside and outside the pass-band, and good temperature characteristics. By employing the filter in a communication apparatus, high performance and low cost communication apparatus can be realized.